In accordance with recent downsizing of mobile communication devices, demand for downsizing of non-reciprocal circuit elements such as isolators or circulators used in the communication devices has increased.
A conventional lumped element type circulator has an assembled circulator element with a circular plane shape and a basic structure as shown in an exploded oblique view of FIG. 1.
In the figure, a reference numeral 100 denotes a circular substrate made of a non-magnetic material such as a glass-reinforced epoxy. Center conductors (inner conductors) 101 and 102 are formed on the top face and next to the bottom face of the non-magnetic material substrate 100, respectively. These inner conductors 101 and 102 are electrically connected with each other by via holes 103 passing through the substrate 100. Circularly shaped members 104 and 105 made of a ferromagnetic material are attached to the both faces of the non-magnetic material substrate 100 having the inner conductors 101 and 102 so that rotating RF (Radio Frequency) magnetic fluxes are induced In these ferromagnetic members 104 and 105 due to an RF power applied to the inner conductors 101 and 102. The conventional circulator element of the circulator has a circular plane shape and is constructed by assembling, namely piling and bonding, the ferromagnetic members 104 and 105 on the both sides of the non-magnetic material substrate 100.
The circulator as a whole is constructed, as shown in its exploded oblique view of FIG. 2, by stacking and fixing in sequence the ferromagnetic members 104 and 105, grounding conductor electrodes 106 and 107, exciting permanent magnets 108 and 109 and a metal housing separated to upper and lower parts 110 and 111 on the both side of the non-magnetic material substrate 100 having the inner conductors 101 (102), respectively. The housing parts 110 and 111 form a magnetic path of the magnetic flux from and to the exciting permanent magnets 108 and 109.
If a RF power Is applied to the inner conductors 101 and 102 through terminal circuits not shown, RF magnetic flux rotating around the inner conductors 101 and 102 will be produced In the ferromagnetic members 104 and 105. Under this state, If a dc magnetic field perpendicular to the RF magnetic flux is applied from the permanent magnets 108 and 109, the ferromagnetic members 104 and 105 present different permeability .mu..sub.+ and .mu..sub.- depending upon rotating sense of the RF magnetic flux, as shown in FIG. 3. A circulator utilizes this difference of the permeability depending upon the rotating sense. Namely, a propagation velocity of the RF signal in the circulator element will differ in accordance with the rotating sense and thus the signals transmitting to the opposite directions will cancel each other, thereby preventing the propagation of the signal to a particular port.
A non-propagating port is determined in accordance with its angle against a driving port due to the permeability .mu..sub.+ and .mu..sub.- of the ferromagnetic member. For example, if ports A, B and C are arranged in this order along a certain rotating sense, the port B will be determined as the non-propagating port against the driving port A and the port C will be determined as the non-propagating port against the driving port B. Terminating one port of thus arranged circulator might constitute an isolator. Termination of the port can be realized by connecting to the port a matched resistor such as a chip resistor, or a thick or thin film resistor formed on a substrate for providing a resonance capacitor.
In such non-reciprocal circuit element, the ratio of volume occupied by the permanent magnet(s) is typically larger than that of another components. This has made difficult to downsize the non-reciprocal circuit element.
Most of conventional lumped element circulators may have a structure represented by an equivalent circuit shown in FIG. 4. In this case, one end (outer conductor) 400 of each inductor of the circulator is directly connected to the ground.
Known in this field is, in order to widen frequency band of a circulator, to insert a serial resonance circuit 501 for adjusting eigen values of in-phase (equal phase) excitation between a common connection point (outer conductor) 500 to which one end of each inductor of the circulator is commonly connected and the ground, as shown in an equivalent circuit of FIG. 5.
In general, to obtain three-port circulator operation, it is necessary to keep those admittances at in-phase excitation, positive phase excitation and negative phase excitation thereof have relationship of angular difference of 120 degrees with each other. The admittances at the positive phase excitation and the negative phase excitation will generally vary depending upon frequency change but admittance at the in-phase excitation will never change. Thus, if the frequency changes greatly, it is impossible to fees the relationship of angular difference of 120 degrees in the admittances causing that circulator operation cannot be expected. As a result, the operation frequency band of the circulator is limited to a narrower band.
Contrary to this, as aforementioned, by additionally inserting the serial resonance circuit for adjusting eigen values of in-phase excitation, the relationship of angular difference of 120 degrees in the admittances can be kept for a long time resulting the operation frequency band of the circulator to widen. However, the addition of the LC serial resonance circuit results of increase in the number of components of the circulator and therefore invites difficulty of downsizing of the circulator. In addition, since it is very difficult to make a small and high-performance inductor, the LC serial resonance circuit to be added will become large in size.
Japanese Patent Publication No.49(1984)-28219 discloses a circulator with capacitors each of which is inserted between one end of each inner conductor and the grounded conductor. An equivalent circuit of this circulator is shown in FIG. 6. As will be understood from the figure, in the circulator, capacitors 601, 602 and 603 are connected to respective ends of three inner conductors. However, according to this structure, these capacitors will exert an influence upon not only eigen values of In-phase excitation but also eigen values of both positive and negative phase excitations. Therefore, as well as the conventional art shown in FIG. 4, when the frequency changes greatly, it is impossible to keep the relationship of angular difference of 120 degrees in the admittances causing that circulator operation cannot be expected. As a result, the operation frequency band of the circulator is limited to a narrower band.
Temperature characteristics of the non-reciprocal circuit element will be discussed hereinafter.
There are various factors that will effect on the temperature characteristics of a non-reciprocal circuit element such as a circulator. It is considered that the main factor is temperature characteristics of saturation magnetization in the ferromagnetic material such as YIG (yttrium iron garnet) used for the circulator element, or the temperature characteristics of the permanent magnet(s) for providing bias magnetic field. In general, change in the temperature characteristics of the ferromagnetic material such as YIG used is larger than that of the bias magnetic field. Thus, the higher the temperature of the circulator, the higher its operation frequency becomes. This causes effective frequency band to be used to become narrower. Thus, in general, gadolinium is substituted in YIG to improve the temperature characteristics of saturation magnetization in YIG. However, the substitution of gadolinium causes loss of YIG to increase and therefore invites increased insertion loss of the circulator. Also, such substitution cannot perfectly adjust the temperature characteristics.
As aforementioned, with the spread of and downsizing of recent mobile communication devices, the non-reciprocal circuit elements themselves are requested to be manufactured in smaller size, in lighter weight and in lower height. In order to satisfy these requirements, it is important to make components of the non-reciprocal circuit element, particularly permanent magnet(s), in smaller size.
The conventional art has another problem that if the non-reciprocal circuit element is made in smaller size, its operation frequency will increase and thus it is difficult to obtain a desired operation frequency.